The E2F1 transcription factor is unique in that it possesses both oncogenic and tumor suppressive properties. During the current funding period for this project, the mechanisms by which E2F1 can both promote and suppress tumor development have been explored using several mouse model systems. Emphasis was placed on characterizing the tumor suppressive activity of E2F1 since this process is poorly understood and may have important clinical implications. It was found that transgenic expression of E2F1 in the epidermis could suppress tumor development in a chemical carcinogenesis assay in which Ras activation serves as the initiating event. This ability to inhibit skin carcinogenesis was unique to E2F1 since other E2F family members and Myc did not have this activity. Through genetic crossing experiments it was discovered that the suppression of skin carcinogenesis by E2F1 required functional p53 and p19Arf tumor suppressor genes. In addition, it was found that inactivation of the endogenous E2fl gene cooperated with the overexpression of c-myc to promote spontaneous tumorigenesis. Interestingly, in both model systems the ability of E2F1 to suppress tumorigenesis did not correlate with the regulation of apoptosis by E2F1. The overall hypothesis of this proposal is that E2F1 stimulates a tumor suppressive pathway that includes ARF and p53 and that functions to inhibit tumorigenesis in response to some oncogenic signals, such as c-myc overexpression or Ras activation, through a non-apoptotic mechanism. To test this hypothesis, various mouse models will be used to molecularly and genetically define the tumor suppressive pathway in which E2F1 operates. Roles for E2F1 in regulating p53 activity, DNA integrity, and premature senescence will be examined. Using information gained from the mouse model studies, the relevance of tumor suppression by E2F1 will be established by searching for E2F1 gene silencing in specific human cancers. Finally, studies will be performed that will begin to translate what is learned from the mouse models into potential cancer therapies aimed at harnessing the tumor suppressive activity of E2F1.